I did simulations in Fluent. The 3D computation domain is constructed by simply repeating the 2D mesh in z-direction (spanwise). Now I want to get the spanwise average of the flow field, can anyone give a hint about how to do it in Tecplot?

Hi,
Go Data --> Extract --> Slice from Plane
Make for example 40 slices and make average of them. Each slice will become a new zone. When averaging, go Equation, write an equation to calcualte the sum and divide by the number of slices.

Potential problem is that you may find on each slice, node location changes, so when adding them together tecplot will give wrong result. In such case, you should try interpoloate. This means interpolate the data on this slice to next slice (initially assigned 0 values), and on the next slice add them together. You will need additional variables to save the data on other slices on that slice. Yes, it is tricky.